bump android target version to 27 (#20843)

Summary:
bump target version to 27
pass all current ci.
none
[GENERAL] [ANDROID] [FEATURE] - bump android target version to 27
Pull Request resolved: facebook/react-native#20843

Differential Revision: D9845999

Pulled By: hramos

fbshipit-source-id: 3c532a2d5a2b7d201bacab54cf3d60e1efffb653

templates/HelloWorld/android/build.gradle

@@ -5,7 +5,7 @@ buildscript {
         buildToolsVersion = "27.0.3"
         minSdkVersion = 16
         compileSdkVersion = 27
-        targetSdkVersion = 26
+        targetSdkVersion = 27
         supportLibVersion = "27.1.1"
     }
     repositories {